Tony Allevato Tony Allevato - 2 months ago 5
C++ Question

Specializing a template on a lambda in C++0x

I've written a traits class that lets me extract information about the arguments and type of a function or function object in C++0x (tested with gcc 4.5.0). The general case handles function objects:

template <typename F>
struct function_traits {
template <typename R, typename... A>
struct _internal { };

template <typename R, typename... A>
struct _internal<R (F::*)(A...)> {
// ...
};

typedef typename _internal<decltype(&F::operator())>::<<nested types go here>>;
};


Then I have a specialization for plain functions at global scope:

template <typename R, typename... A>
struct function_traits<R (*)(A...)> {
// ...
};


This works fine, I can pass a function into the template or a function object and it works properly:

template <typename F>
void foo(F f) {
typename function_traits<F>::whatever ...;
}

int f(int x) { ... }
foo(f);


What if, instead of passing a function or function object into
foo
, I want to pass a lambda expression?

foo([](int x) { ... });


The problem here is that neither specialization of
function_traits<>
applies. The C++0x draft says that the type of the expression is a "unique, unnamed, non-union class type". Demangling the result of calling
typeid(...).name()
on the expression gives me what appears to be gcc's internal naming convention for the lambda,
main::{lambda(int)#1}
, not something that syntactically represents a C++ typename.

In short, is there anything I can put into the template here:

template <typename R, typename... A>
struct function_traits<????> { ... }


that will allow this traits class to accept a lambda expression?

Answer

I think it is possible to specialize traits for lambdas and do pattern matching on the signature of the unnamed functor. Here is the code that works on g++ 4.5. Although it works, the pattern matching on lambda appears to be working contrary to the intuition. I've comments inline.

struct X
{
  float operator () (float i) { return i*2; }
  // If the following is enabled, program fails to compile
  // mostly because of ambiguity reasons.
  //double operator () (float i, double d) { return d*f; } 
};

template <typename T>
struct function_traits // matches when T=X or T=lambda
// As expected, lambda creates a "unique, unnamed, non-union class type" 
// so it matches here
{
  // Here is what you are looking for. The type of the member operator()
  // of the lambda is taken and mapped again on function_traits.
  typedef typename function_traits<decltype(&T::operator())>::return_type return_type;
};

// matches for X::operator() but not of lambda::operator()
template <typename R, typename C, typename... A>
struct function_traits<R (C::*)(A...)> 
{
  typedef R return_type;
};

// I initially thought the above defined member function specialization of 
// the trait will match lambdas::operator() because a lambda is a functor.
// It does not, however. Instead, it matches the one below.
// I wonder why? implementation defined?
template <typename R, typename... A>
struct function_traits<R (*)(A...)> // matches for lambda::operator() 
{
  typedef R return_type;
};

template <typename F>
typename function_traits<F>::return_type
foo(F f)
{
  return f(10);
}

template <typename F>
typename function_traits<F>::return_type
bar(F f)
{
  return f(5.0f, 100, 0.34);
}

int f(int x) { return x + x;  }

int main(void)
{
  foo(f);
  foo(X());
  bar([](float f, int l, double d){ return f+l+d; });
}